How do you turn softwood to hardwood?

Sadly you can't. A tree will grow and will be catagorised into it's species (Oak, Radiata Pine, Sassafras). Each of these species is naturally a hardwood or a softwood. It is not possible to turn the tree from one species to another; and from softwood to hardwood. As a side note, if you have an area of wood made from a softwood e.g floor boards, giving them several coats of an oil lacquer (clear varnish) will provide an added strength against immediate dents like high heels.